The principle of operation of an inductive sensor is based on the variation in coupling between a primary winding and secondary windings of a transformer operating at high frequency and without the use of a magnetic circuit. The coupling between these windings varies as a function of the position of a movable conductor (of electricity) piece, generally known as a “target”. Currents induced in the target will in fact modify the currents induced in the secondary windings. By adapting the configuration of the windings and knowing the current injected in the primary winding, measuring the current induced in the secondary windings makes it possible to determine the position of the target.
Document EP0182085, incorporated herein by reference, describes the principle of such an inductive sensor.
In order to integrate such an inductive sensor in a device, especially an electronic device, it is known how to make the aforementioned transformer on a printed circuit card. The primary winding and the secondary windings are then formed by tracks traced on the printed circuit card. The primary winding is then energized for example by an external high-frequency source and the secondary windings are then the site of currents induced by the magnetic field created by the circulation of a current in the primary winding. The target, which is a conductive piece, such as metal, can have a simple shape. For example, it may be a piece cut out from a metal sheet. In order to make a linear sensor, the cutout to form the target is for example rectangular, while for a rotary sensor this cutout will be for example in the form of an angular sector with radius and angle suited to the movement of the piece.
Generally, two sets of secondary windings are designed to accomplish sine and cosine functions of the target position in one complete run of the sensor. Such functions (cos and sin) are well known and can be easily processed by an electronic system. By forming the ratio of the sine to the cosine and then applying an arc tangent function, one obtains an image of the position of the target. The argument of the sine and cosine functions is a linear (or affine) function of the position of the target whose course then represents a more or less large portion of the spatial period of these trigonometric functions.
From a physical standpoint, the modification of the coupling between the primary circuit and the secondary circuits is realized thanks to the phenomenon of the electromagnetic skin effect, known to the person skilled in the art. The primary circuit being energized by a high-frequency source, the phenomena taking place in the entire sensor are high-frequency phenomena. The target whose position is to be found is a massive conductor piece and is the site of significant induced currents. The depth of penetration of these induced currents is relatively shallow (hence the term skin). For example, it is in the order of 50 μm in the case of an aluminum target. Thus, the induction does not penetrate into the target and the magnetic flux produced by the primary circuit thus bypasses the target. Due to this fact, the field lines are strongly modified. This modification is perceived by the secondary circuits which, depending on the position of the target, receive more or less flux. These fluxes are variable as a function of the position of the target and also variable as a function of time and therefore generate a voltage at the terminals of the secondary circuits. By measuring these voltages, one thus obtains a signal which, when analyzed, lets one know the position of the target.
When it is not possible to place a sensor at the end of a shaft to determine the angular position of that shaft, it is known how to provide the shaft with a helix which is placed opposite a linear sensor. In fact, if one looks at a helix in rotation relative to a fixed point, one sees from this fixed point a surface in axial displacement. Thus, it is as if the target were moving linearly in front of the sensor.
Thus, a linear sensor can give an indication as to the angular position of a shaft by adapting the shape of the target. However, when the shaft whose angular position is to be determined is moving axially, even if it is only parasitic movements, the angular measurement is falsified on account of this axial movement.